This invention relates to improvements in the forming of ductile sheet metals into bodies of irregular shapes. It is primarily applicable to forming those materials which have a high ductility and also a low flow stress, although it may be applied also to normal materials which can be rendered sufficiently easy to stretch at elevated temperatures.
U.S. Pat. No. 837,297 teaches a method of producing shell or trough shaped cases or articles of sheet metal by first bending a sheet of metal without drawing or stretching the same and thereafter subjecting the sheet to a drawing or stretching action until the sheet of metal is brought into the shape of the desired article. This method is carried out by a male die forcing the sheet of metal into a female die cavity.
U.S. Pat. Nos. 2,783,727 and 2,783,728 teach sheet metal forming by use of male and female dies having a cushion positioned between the female die and sheet of metal. A standard hydraulic press is used to drive the male die member against the sheet of metal and the cushion.
U.S. Pat. No. 4,045,986 teaches the shaping of a ductile metal sheet by the application of pressure differentials, the peripherally clamped sheet is first urged in one direction by differential pressure against a female molding surface to form a partially shaped bubble like preform and then a male mold is advanced towards the sheet from the opposite side and a reverse pressure differential is applied to cause the preform to conform to the shape of the mold.
The simple process of hydroforming (or otherwise forming by differential pressure) into a female mold is adequate for smoothly rounded shapes that are not too deep, but tends to result in products having appreciable variations in thickness. Forming onto a male mold, either by hydroforming or other differential pressure techniques, or by advancing a male mold member into a peripherally clamped flat sheet or bubble, is also unacceptable because the part blank tends to wrinkle before any forming action can occur.
There has not been an entirely satisfactory tooling system or method of forming an initially flat, peripherally clamped, sheet of ductile sheet metal which will allow the production of relatively deep and complex shapes, while keeping to a minimum the faults hereinbefore mentioned until the emergence of the instant invention.